As to methods for removing detrimental substances such as sulfur oxide, nitrogen oxide, hydrogen chloride and cyanogen contained in a combustion waste gas produced from an incinerator, for example, for incinerating industrial and household wastes, many research reports have so far been released, and some of them have already been industrialized.
However, research efforts have been started only recently as to methods for removing, from the above-mentioned combustion waste gas, organic chlorine compounds, though slight in amount, having a strong toxicity, such as polychlorinated dibenzo-p-dioxins, polychlorinated dibenzofurans, polychlorinated biphenyl and chlorophenol contained in the combustion waste gas. A method for removing such organic chlorine compounds from the combustion waste gas in an industrial scale has not therefore as yet been established.
Polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans are stable substances and are insoluble in water, so that the toxicity thereof is semipermanently retained. In particular, polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans are very detrimental substances which pollute the environment by their strong toxicity.
Polychlorinated dibenzo-p-dioxins are classified, in terms of the number of chlorines, into dichloride, tetrachloride, pentachloride and hexachloride, and there are more than 70 types of isomers. Among these polychlorinated dibenzo-p-dioxins, tetrachlorinated dibenzo-p-dioxin has the strongest toxicity.
For the purpose of removing detrimental organic chlorine compounds such as polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans from a combustion waste gas discharged from an incinerator, the following methods are known:
(1) Removing method based on combustion:
This method comprises the steps of: increasing the temperature in an incinerator to at least 1,000.degree. C., and causing a decomposition reaction of organic chlorine compounds contained in a combustion waste gas under the effect of this high temperature, thereby removing the organic chlorine compounds from the combustion waste gas.
An alternative method comprises the steps of: introducing a combustion waste gas discharged from an incinerator into another combustion furnace, and causing a decomposition reaction of organic chlorine compounds contained in the combustion waste gas at a temperature of at least 1,000.degree. C., thereby removing the organic chlorine compounds from the combustion waste gas.
(2) Removing method based on adsorption:
This method comprises the steps of: passing a combustion waste gas discharged from an incinerator through an adsorbent such as activated charcoal and causing the adsorption by the adsorbent of organic chlorine compounds contained in the combustion waste gas for removal.
(3) Removing method based on washing:
This method comprises the step of: washing with a chemical solution a combustion waste gas discharged from an incinerator to remove organic chlorine compounds contained in the combustion waste gas.
The removing method based on combustion mentioned in (1) above has the following problems:
(a) In order to remove organic chlorine compounds from the combustion waste gas in the incinerator, it is necessary to increase the temperature in the incinerator to at least 1,000.degree. C. Usually, however, the temperature in the incinerator is within a range of from about 800.degree. to about 900.degree. C. In order to increase the temperature in the incinerator to at least 1,000.degree. C., therefore, it is necessary to totally reconstruct the incinerator.
(b) Most of the incinerators currently in use are stoker furnaces, and low-temperature portions may easily be locally produced in such a stoker furnace. It is therefore difficult to increase the temperature in the furnace to at least 1,000.degree. C. throughout the entire furnace interior
(c) Increasing the temperature in the incinerator may easily damage the furnace wall as a result of melting of ash in the incinerated product.
(d) Installation of another combustion furnace for removing the organic chlorine compounds from the combustion waste gas requires higher equipment and running costs. Furthermore, the low contents of the organic chlorine compounds in the combustion waste gas lead to a low removing efficiency of the organic chlorine compounds.
The removing method based on adsorption mentioned in (2) above has the following problem:
(a) It is necessary to take a measure for regenerating the adsorbent such as activated charcoal after adsorbing the organic chlorine compounds.
The removing method based on washing mentioned in (3) above has the following problem:
(a) It is necessary to take a measure for eliminating toxicity of the waste liquid after washing the combustion waste gas.
As a method for solving the above-mentioned problems, Japanese Patent Provisional Publication No. 63-290,314 dated Nov. 28, 1988 discloses a method for removing organic chlorine compounds from a combustion waste gas discharged from an incinerator, which comprises the step of:
bringing organic chlorine compounds contained in a combustion waste gas discharged from an incinerator into contact with a catalyst comprising platinum supported on the surface of a carrier made of ceramics at a temperature within a range of from 300.degree. to 900.degree. C. to cause a decomposition reaction of the organic chlorine compounds, thereby removing the organic chlorine compounds from the combustion waste gas (hereinafter referred to as the "prior art 1").
Japanese Patent Provisional Publication No. 2-35,914 dated Feb. 6, 1990 discloses a method for removing organic chlorine compounds from a combustion waste gas discharged from an incinerator, which comprises the step of:
bringing organic chlorine compounds contained in a combustion waste gas discharged from an incinerator and having a temperature of at least 150.degree. C. into contact with a catalyst comprising at least one selected from the group consisting of titanium oxide, vanadium oxide, tungsten oxide, platinum and palladium to cause a decomposition reaction of the organic chlorine compounds, thereby removing the organic chlorine compounds from the combustion waste gas (hereinafter referred to as the "prior art 2").
The above-mentioned prior art 1 has the following problems: In the prior art 1, it is necessary to bring the combustion waste gas into contact with the catalyst at a high temperature within the range of from 300.degree. to 900.degree. C. The waste gas discharged from the incinerator and subjected to dust collection through a dust collector has usually a temperature of up to 350.degree. C. In the prior art 1, therefore, it is necessary in most cases to heat the combustion waste gas to the above-mentioned high temperature, thus requiring a heating equipment of the combustion waste gas. This results in higher equipment and running costs. Because of the easy deterioration of the catalyst under the effect of the high-temperature combustion waste gas, furthermore, it is impossible to remove the organic chlorine compounds from the combustion waste gas stably for a long period of time.
The above-mentioned prior art 2 has the following problem; The prior art 2 gives a removing ratio of polychlorinated dibenzo-p-dioxins from the combustion waste gas within a range of from about 22 to 38%, and a removing ratio of polychlorinated dibenzofurans from the combustion waste gas within a range of from about 46 to 49%. In the prior art 2, therefore, the removing ratios of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans are low.
Under such circumstances, there is a strong demand for the development of a method which permits removing detrimental organic chlorine compounds such as polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans contained in a combustion waste gas discharged from an incinerator at a high efficiency and stably for a long period of time by means of a catalyst without heating the combustion waste gas, but such a method has not as yet been proposed.
An object of the present invention is therefore to provide a method which permits removing detrimental organic chlorine compounds such as polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans contained in a combustion waste gas discharged from an incinerator at a high efficiency and stably for a long period of time by means of a catalyst, without heating the combustion waste gas.